opengl Instancing Instancing by Vertex Attribute Arrays


Example

3.3

Instancing can be done via modifications to how vertex attributes are provided to the vertex shader. This introduces a new way of accessing attribute arrays, allowing them to provide per-instance data that looks like a regular attribute.

A single instance represents one object or group of vertices (one grass leaf etc). Attributes associated with instanced arrays only advance between instances; unlike regular vertex attributes, they do not get a new value per-vertex.

To specify that an attribute array is instanced, use this call:

glVertexAttribDivisor(attributeIndex, 1);

This sets vertex array object state. The "1" means that the attribute is advanced for each instance. Passing a 0 turns off instancing for the attribute.

In the shader, the instanced attribute looks like any other vertex attribute:

in vec3 your_instanced_attribute;

To render multiple instances, you can invoke one of the Instanced forms of the value glDraw* calls. For example, this will draw 1000 instances, with each instance consisting of 3 vertices:

glDrawArraysInstanced(GL_TRIANGLES, 0, 3, 1000); 

Instanced Array Code

Setting up VAOs, VBOs and the attributes:

// List of 10 triangle x-offsets (translations)
GLfloat translations[10];
GLint index = 0;
for (GLint x = 0; x < 10; x++)
{
    translations[index++] = (GLfloat)x / 10.0f;
}

// vertices
GLfloat vertices[] = {
     0.0f,   0.05f,
     0.05f, -0.05f, 
    -0.05f, -0.05f,
     0.0f,  -0.1f,
};

// Setting VAOs and VBOs
GLuint meshVAO, vertexVBO, instanceVBO;
glGenVertexArrays(1, &meshVAO);    
glGenBuffers(1, &instanceVBO);
glGenBuffers(1, &vertexVBO);

glBindVertexArray(meshVAO);

    glBindBuffer(GL_ARRAY_BUFFER, vertexVBO);
    glBufferData(GL_ARRAY_BUFFER, sizeof(vertices), vertices, GL_STATIC_DRAW);
    glEnableVertexAttribArray(0);
    glVertexAttribPointer(0, 2, GL_FLOAT, GL_FALSE, 2 * sizeof(GLfloat), (GLvoid*)0);

    glBindBuffer(GL_ARRAY_BUFFER, instanceVBO);
    glBufferData(GL_ARRAY_BUFFER, sizeof(translations), translations, GL_STATIC_DRAW);
    glEnableVertexAttribArray(1);
    glVertexAttribPointer(1, 1, GL_FLOAT, GL_FALSE, sizeof(GLfloat), (GLvoid*)0);
    glVertexAttribDivisor(1, 1); // This sets the vertex attribute to instanced attribute.

    glBindBuffer(GL_ARRAY_BUFFER, 0);

glBindVertexArray(0);

Draw call:

glBindVertexArray(meshVAO);
glDrawArraysInstanced(GL_TRIANGLE_STRIP, 0, 4, 10); // 10 diamonds, 4 vertices per instance
glBindVertexArray(0);

Vertex shader:

#version 330 core
layout(location = 0) in vec2 position;
layout(location = 1) in float offset;

void main()
{
    gl_Position = vec4(position.x + offset, position.y, 0.0, 1.0);
}

Fragment shader:

#version 330 core
layout(location = 0) out vec4 color;

void main()
{
    color = vec4(1.0, 1.0, 1.0, 1.0f);
}